1. Field of the Invention
This invention relates to a temperature compensation circuit which is particularly useful in compensating for changes in ambient temperature of pressure and temperature sensors. Sensors generally have a thermal response such that operation at a temperature other than that at which the sensor has been calibrated results in an error in the output of the sensor. For example, strain gages are often employed in a pressure sensor where the medium communicating with the gages is a liquid which has a temperature different from that at which the gages and resultant sensor were calibrated. Similarly, thermocouples used in accurately measuring temperature changes may have two junctions of dissimilar metals away from the medium in which the temperature is to be sensed. The temperature in these two junctions may vary independently of the temperature of the medium and may also be different from the temperature at which the thermocouple was calibrated. As a consequence spurious and erroneous electromotive forces or voltages are generated which distort the output of the thermocouple.
2. Description of the Prior Art
Various methods and apparatus have been employed in the past to compensate for the thermal responsiveness of sensors. For example, wire wound resistors employing low temperatures coefficient wire, as close to zero coefficient as possible, have been employed. Additionally, where the gages have a positive or negative temperature coefficient, a wire wound resistor having the opposite temperature coefficient and hopefully the same thermal slope or resistance variation with respect to temperature was connected in series with the gages. However, these wire wound resistors have a tendency to break due to the differences in thermal expansion rates of the resistors and the conformal coating used to hold them in place. Consequently these wire wound resistors tend to be unreliable. Alternatively, thermistors have been employed in compensation circuits but the thermistors exhibit a nonlinear temperature coefficient that is not compatible with the thermal slope of the sensors to be compensated. Moreover the temperature compensation circuits are generally useful with only one specific sensor requiring a specific amount of compensation at a particular temperature which severely limits the usefulness of these compensation circuits.